Carbon Nanotube-Based Sensor Platform for Bioelectronic Nose - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 227

olfactory receptors, adenylyl cyclases, and ion channels induced the flux of calcium
ions into the nanovesicles. The influx resulted in the accumulation of calcium ions.
The accumulated calcium ions gave a positive field effect on the underlying CNT-
FET, thus resulting in the decrease of conductance of the CNT-FET.
Based on this mechanism, the NBN detected amylbutyrate (AB), a specific odor-
ant of hOR2AG1, down to 1 fM concentration (Fig. 12.7b ). The validity of the
mechanism can be supported by the following control experiments. One experiment
was conducted using nanovesicles without hOR2AG1, whereas another experiment
was performed in calcium-free phosphate buffered saline (PBS). The other experi-
ment was conducted in PBS with MgCl 2 which blocks ion channels. In those experi-
ments, there were no changes in the conductance of CNT-FETs. This indicates that
olfactory receptors, calcium ions, and ion channels are critical components for the
operation of the NBNs. Also, this shows that the NBN platform could mimic the
olfactory signal transduction in cells.
Figure 12.7c shows the normalized sensitivity of NBNs after the introduction
of odorant molecules with various concentrations. Here, the conductance change
of a NBN was measured after the introduction of odorant solutions with different
concentrations, and the conductance change was normalized by its maximum to
calculate the normalized sensitivity. Note that the conductance change increased as
the concentration of AB increased and saturated at a 10 nM concentration, while the
others did not show any significant change.
Figure 12.7d shows a real time conductance measurement from an NBN device
after the introduction of pentylbutyrate (PB), pentylvalerate (PV), butylbutyrate
(BB), and AB. Conductance changes were negligible after injecting 1 µM of non-
target odorants such as PB, PV, and BB. On the other hand, after the injection of
AB with 100 pM, the significant conductance change was observed. This result
indicates the NBNs are highly selective to target molecules with a single-carbon-
atomic resolution.

12.3.3 Canine Receptor-Based Bioelectronic Nose

Taking advantages of the high sensitivity and the high selectivity of canine olfac-
tory systems, Park et al. developed a canine receptor-based sensor that mimicked
canine nose responses for the sensitive and selective detection of hexanal, an in-
dicator of the oxidation of food [ 37 ]. Figure 12.8a shows a scheme depicting the
structure of a canine receptor-based sensor. This sensor was composed of a CNT-
FET and canine olfactory nanovesicles immobilized on the CNT-FET. In this sen-
sor, the binding of hexanal to canine olfactory receptors (cfOR5269) successively
activated G proteins, adenylyl cyclases, and Ca 2+ channels following the cAMP
pathway in the nanovesicle. The activation of Ca 2+ channels generated the influx
of Ca 2+ , which increased the potential of the nanovesicle in the vicinity of CNTs.
Since CNT channels exhibited p-type characteristics under ambient conditions, the
increased potential of the nearby nanovesicle resulted in the decrease of the CNT
channel conductance. In this bioelectronic nose, the binding of hexanal onto canine